Fused silica tubes are often used for shrouding molten metal during continuous casting. These tubes are made either by slip casting or by injection molding; see for example U.S. Pat. No. 4,011,299 and W. German Patent No. 26,33,309. The main advantage of such tubes is excellent resistance to thermal shock. Their service life, however, is limited to a few heats because of wear caused by devitrification, pyroplastic flow of silica, reduction of silica by elements in the steel, and corrosive attack by molten mold powder.
In an effort to increase service life, the steel industry has recently tried shroud tubes manufactured from an alumina-graphite composition. These tubes contain from 15 to 30 weight percent flake graphite, and are made by isostatic pressing. These tubes are resistant to the erosive action of molten steel and to the reducing elements in the steel (Al, Mn, Si). Their main disadvantages, however, are high manufacturing cost, a tendency to form a network of alumina crystals along the bore during service which impedes metal flow, and susceptibility to corrosive attack by mold powder.
Improvements have been recently made to increase the mold powder resistance of these tubes by addition of zirconia-containing compounds to the formulation. Also, efforts have been made to limit the tendency to form alumina growths by adding lime or dolomite to the formulation. However, a serious drawback remains--the high cost of manufacture of such tubes. Isopressing is a very capital and labor intensive means of production. Often the tubes require machining on a lathe after isopressing to achieve dimensional specifications.
As an alternative to manufacturing the tubes by isopressing, it would be desirable to form them by injection molding techniques. Injection molding is desirable because of its relatively low capital investment and its high rate of production. However, because of the non-wetting nature of flake graphite, injection molding of alumina-graphite compositions requires extremely high binder levels regardless of the molding vehicle used. Use of high binder levels results in undesirable physical properties. Laboratory experiments showed that binder levels of at least 20% were necessary in mixes which contained flake graphite before an injectable consistency was achieved. Many attempts were made to mask the surface of flake graphite by encapsulating the graphite in a film of resin, gelatin or by coating the surface with silicon carbide. None of these attempts were successful in decreasing the binder requirement.
Additionally, in conventional injection molding processes, waxes or low carbon-yielding resins, such as polystyrene or polyethylene, are chosen as molding vehicles because they are solids at room temperature and become liquid at elevated temperatures. The main disadvantages of using these vehicles are that they do not provide a permanent bond. The shape must be heated to very high sintering temperatures to initiate bonding.
In view of the deficiencies in utilizing graphite and low carbon-yielding resins heretofore enumerated, it was decided that a new composition should be formulated for manufacturing shroud tubes. Many different types of coarse carbon were tested; however, only calcined fluidized bed coke could be used in injection molding processes without requiring extremely high binder levels. It was found that as little as 10% binder was needed to produce a moldable consistency when calcined fluidized bed coke was used in a mix instead of flake graphite. This discovery was quite unexpected since it is generally understood that carbon cannot be wetted. It is suspected that the spherical structure of calcined fluidized bed coke allows it to be easily wetted.
Fine silicon metal was added to the mix to protect the carbon from oxidation and to react with fine carbon to form secondary silicon carbide during coking, which enhances the strength of the tube. Furthermore, high carbon yielding resins were used as the molding vehicles. These types of resins, if pyrolysized under reducing conditions, produce a carbon char which is known to provide excellent strength in refractories, without the need for high temperature sintering.